The present invention concerns a sintered article, and a method of producing the sintered article. The method and article are particularly useful in making, and as, a low equivalent series resistance (ESR) capacitor anode precursor of an oxidized valve metal.
Sintering is a well-known method for making articles from particles of a material. The particles may be coarse or very finely divided, even in the foam of a powder. Typically, the material to be sintered is formed into a particular shape, for example, by compression, within a mold, using a press, or by another known method. The shaped object, prepared from the particles of the material, is referred to here as a green body. The green body is heated to a temperature below the melting point of the material, with the result that the particles of the green body adhere to each other in what is here called a sintered body. This heating step is referred to here as the sintering process.
The materials that may be employed in a sintering process include metals, ceramics, and many other materials. The green body may be subjected to compression during the sintering process, or may simply be heated, without the application of any external pressure, in the sintering process. The ambient in which the sintering occurs may be air, a controlled gas or mixture of gases, or a vacuum, depending upon the reactivity of the material and the desired structure of the sintered body. For example, in some circumstances the exposed surfaces of the particles in the sintered body should be free of oxides or other compounds that might be formed in sintering the green body in a particular ambient. A binder may be used to keep the particles of the green body together before sintering. The binder may be driven off or reacted in the sintering process or removed with a solvent or by a different process after completion of the sintering process.
In the sintering process at least one of the dimensions and shape of a green body usually changes. In most instances, the density of sintered body is higher than the density of the green body. Although the material being sintered is not fused, the material of the particles being sintered may move into the voids within the green body in the sintering process. Even assuming that none of the material of the green body is lost during the sintering process, the overall volume of the green body usually decreases, i.e., the porosity decreases, increasing the density of the sintered body, as compared to the density of the green body. Material transport phenomena, such as recrystallization, diffusion, and evaporation and condensation, may occur in the sintering process. In any event, the green body and its external dimensions usually shrink in the sintering process. The shrinkage must be taken into account when forming a green body that is expected to produce a sintered body of specific external dimensions and shape.
Still other phenomena are observed in sintering when a green body contains all or part of a solid body. As used here, the term “solid body” means a body made of a material having its nominal density as an element, a compound, a mixture, an alloy, and the like. A solid body does not appreciably shrink during sintering of particles of a material in which at least part of the solid body is embedded. If the solid body is a material having some porosity and shrinks to some degree in the sintering process, then the shrinkage of the “solid body” is negligible with respect to the shrinkage of the green body in the sintering process.
In a green body including a solid body, the particles of the green body are compressed or formed to encapsulate at least part of the solid body. The solid body may be, in some applications, a wire or a rod that projects from inside the green body to outside the green body. The wire or rod may provide a handle for handling the green body and the sintered body without the necessity of touching either body. The wire or rod may also provide an external electrical connection to the sintered body without the necessity of attaching, after sintering, a wire or rod to the sintered body. In some electrical applications, for example, in a conventional anode of a capacitor, an electrical lead may be attached to a sintered body of a valve metal, for example of tantalum, by welding. That additional step and its complications may be avoided if the wire can be embedded in the valve metal particles before their sintering. In the sintering process, the wire bonds to the sintered particles.
When a solid body is present within a green body, the solid body affects the sintering process and the configuration of the sintered body as compared to the result of sintering of a similar green body free of any solid body. The presence of the solid body changes, through at least part of the green body, the composition and porosity of the green body. The density and porosity become non-uniform in the green body and discontinuous in cross-sections of the green body that intersect the solid body. Therefore, the processes of material transport during sintering that result in changes in density, porosity, dimensions, and shape during sintering become locally variable within the green body. The exterior dimensions of the green body may change in a different way from the changes experienced when the green body is homogeneous.
The non-uniformities within a green body in which a solid body is embedded mean that the sintering particles exert a stress on the solid body in the sintering process. In general, the applied stress is a compressive stress as the green body shrinks while the solid body does not appreciably change in dimensions, density, or porosity. If the material of the particles being sintered is different from the density of the material of the solid body that is at least partially within the green body, additional stresses can be exerted due to differences in coefficients of thermal expansion of the particles and of the solid body. Of course, even when the particles of the green body and the solid body are made from the same material, the coefficients of thermal expansion of the particles and the solid body may be effectively different because of the different densities of the green body, considering the interstices that are present in the green body, but which are effectively absent from the solid body.
The changes in dimensions of a sintered body produced in sintering a green body containing a solid body, as compared to a green body that is relatively homogeneous, are particularly apparent when the sintered body is relatively thin. In this description, the thinness of a body is described with respect to an aspect ratio. Assuming a relatively thin sintered body is generally planar, the area of the sintered body can be obtained by projecting the sintered body onto a plane, in a direction along the thickness direction of the sintered body. The body can be described as having an aspect ratio, based on that area and the thickness of the sintered body. The aspect ratio is the result of dividing a dimension of the area of the sintered body, derived from the area projected, by the thickness of the sintered body. For example, the dimension may be a side of a body that has a generally rectangular area, the length of a diagonal dimension of a body with a generally rectangular area, or the diameter of a generally circular body. The thickness dimension is measured along a direction transverse to that dimension related to the area, for example, perpendicular to the plane upon which a generally, but not perfectly, planar thin sintered body is projected.
A larger aspect ratio green body indicates a thinner green body and a thinner sintered body. Thus, a higher aspect ratio sintered body made from a green body that is not mechanically restrained in the sintering process is more likely to deviate from perfect planarity, as a sintered body, than is a sintered body with a lower aspect ratio. This result follows because stresses induced in sintering more easily distort a thin body. When a solid body is present in a thin planar green body, the probability that a thin sintered body produced from the thin green body will not be planar is increased. The additional sintering stress induced by the solid body are likely not symmetrical with respect to the area of the sintered body, causing curling or bending of the thin green body during the sintering process.
An example of a relatively thin green body, incorporating a solid body, is illustrated in a plan view in FIG. 1A. The sintered body that results from sintering the green body of FIG. 1A is shown in plan view in FIG. 1B. In this example, the sintered body is a valve metal anode precursor intended for use as an anode, after subsequent processing, in a capacitor. Examples of such capacitors are electrolytic wet-slug capacitors and hybrid capacitors described in my U.S. Pat. No. 5,369,547, the contents of which are incorporated herein by reference. While the preferred anode is constructed by sintering particles of tantalum, similar anodes can be made by sintering other valve metals, including niobium, titanium, zirconium, and aluminum.
As shown in FIG. 1A, the green body 1, compressed from metallic tantalum particles, has a generally square shape with four sides 2. The green body 1 also includes a wire 3 projecting from the green body and arranged generally diagonally with respect to the “square” green body. The wire 3 projects from the green body at the intersection of two adjacent sides 2. The wire functions, as explained below, as an electrical connection in completing the anode and in an assembled capacitor. The wire also functions as a handle for post-sintering processing of the anode precursor.
FIG. 1B illustrates the changed shape of a sintered body 4 after the sintering of the green body 1. As indicated by the arrows, there is shrinkage of all sides 2 of the green body in the sintering process to the sides 5 of the sintered body 4. As shown in FIGS. 1A and 1B, the wire 3 is placed along only about one-third to one-fourth of the diagonal of the green body 1. This limited penetration of the wire is maintained to limit distortion and deformation of the green body during sintering due to the presence of that solid body, i.e., the wire 3 in this instance. If the wire were inserted farther into the green body or entirely across the diagonal of the green body, the deformation of the sintered body from a plane could become dramatic. The resulting sintered body can deviate substantially from a planar configuration, sometimes curling enough to resemble the shape of a potato chip or the surface of a saddle. It is important in assembling a capacitor with a sintered anode that the anode be planar to ensure compactness of the capacitor and proper functioning of the anode in cooperation with other components of the capacitor, for example, a cathode and ion-permeable separator.
The inability to produce a planar sintered anode precursor reliably, if the wire 3 extends further into the green and sintered bodies, has an adverse effect upon the performance of the capacitors that are produced using sintered anodes. In the precursor of FIG. 1B, contact between the wire 3 and the sintered body occurs only along the surface of the wire that is within the green and sintered bodies. The resistance of the connection, which is a significant component of the ESR of a capacitor employing an anode prepared from the sintered body 4, directly depends upon the mutually contacting surface areas of the wire and the sintered body. A higher ESR indicates shorter useful capacitor lifetime and lower useful frequency response of the capacitor.
FIGS. 2 and 3 are, respectively, schematic exploded and cross-sectional views of the structure of a capacitor employing two sintered valve metal anodes, made from green bodies, like the anode precursors shown in FIGS. 1A and 1B.
The capacitor 30 shown in FIGS. 2 and 3 can have essentially any shape, in a plan view, transverse to the view of FIG. 3, for example a circular or polygonal shape. An example of the latter shape is shown in FIG. 2. The capacitor 30 includes a case 32, preferably a conductive metal. The case 32 is sealed by and to a header 34. The header 34 is preferably welded to the case.
In the capacitor of FIGS. 2 and 3, three cathodes 36 are alternately laminated with two anodes 38. Each cathode is preferably a metal foil. The anodes 38 are sintered bodies of a valve metal, preferably tantalum, that forms a native oxide on the surfaces of the particles. The oxide thickness may be anodically increased to increase the voltage rating of the capacitor. Respective ion-conductive separators 40 are interposed between the opposing faces of paired cathodes and anodes. The lowermost cathode 36 within the case 32 is in contact with a separator 40. Each of the cathodes includes a projecting electrically conductive tab 42, with an insulator part 44. The insulating part 44 is folded against the inside surface of the case 32 and the tab 42 is electrically connected to the case so that the case forms one electrical terminal, namely the negative terminal, of the capacitor. Likewise, each of the anodes includes a projecting lead 46 that is bent toward the header 34, but electrically insulated from the case 32. The projecting leads 46 pass through a diagonal slot 48 in an electrically insulating polymeric spacer 50. The insulating spacer is further separated from the uppermost electrodes in the case 32 by a separator 40 and an insulating sheet 54 that insulates the anode leads from the adjacent cathode. Additional electrically insulating sheets 56 and 57 are interposed between the spacer 50 and the header 34. The sheets 56 and 57 include a central hole for passage of a pin 58 that functions as the second terminal, namely the positive terminal, of the capacitor. The pin 58 is held in an opening of the header 34 by a glass-to-metal seal 60. The leads 46 from anodes 38 are wound about and bonded to the lower end of the pin 58, inside the case 32. An annular gasket or spacer 6, located between the header 34 and the insulating sheet 57, cushions the assembly while the header 34 is welded to the case 32. Similarly, an insulating band 64, which may be endless or have overlapped ends, is placed in the case 32. The band 64 insulates the anodes from the case 32 and from the cathodes 36 and the cathode leads 44.
A post 68, see FIG. 3, which provides a negative terminal of the capacitor, is optionally welded to the header 34. The header 34 also includes a further opening 70, which is sealed in the completed capacitor. The opening is employed for vacuum infusion of a liquid or gel electrolyte into the case 32 of the capacitor, so that the electrolyte is in contact with the cathodes 36, the anodes 38, and the separators 40.
A structure of the capacitor 30 shown in FIG. 3 that is important in the context of the invention described below relates to the projecting leads 46 that extend from each of the anodes 38. As shown in FIG. 3, and explained above, each of those projecting leads extends from the respective anode along a length necessary to reach and be connected to the positive terminal 58 of the illustrated capacitor. These leads are relatively long and their lengths add to the ESR of the capacitor.